Method of calculating magnetic interaction of molecules using localized magnetic orbital

ABSTRACT

A magnetic interaction is calculated by expressing a localized magnetic orbital as a linear combination of molecular orbitals and determining a localized magnetic orbital to be calculated from a maximum overlap condition evaluated between a reference orbital localized in anatomic orbital and the localized magnetic orbital. Another magnetic interaction is calculated by expressing a localized magnetic crystal orbital as a linear combination of crystal orbitals and determining a localized magnetic crystal orbital to be calculated from the maximum overlap condition.

BACKGROUND OF THE INVENTION

[0001] 1) Field of the Invention

[0002] The present invention relates to a technology for calculating magnetic interaction of molecules, crystalline compounds, or high polymer compounds each exhibiting ferromagnetism.

[0003] 2) Description of the Related Art

[0004] Conventionally, an evaluation theory of magnetic characteristics has developed by a Heisenberg model which targets at a localized electron system and a Stoner model which targets at an itinerant electron system. However, since the Heisenberg model executes rough approximation and cannot be easily handled, the Heisenberg model is culled out by the Stoner model.

[0005] The Stoner model has passed through a physical examination in a Hubbard model, an Anderson model, or the like which are phenomenological models and, at present, has settled to a band calculation method based on a density functional theory. In the recent band calculation method based on the density functional theory, an effective exchange interaction energy can be obtained at calculation accuracy which is satisfactory to some extent (Sakuma Akimasa, Hitachi Metals Technical Journal, 16, 55 to 60 (2000)).

[0006] However, the conventional evaluation method (band calculation method) targets, for calculation, at an itinerant electron system which uses a Bloch function as a base, and therefore a localized electron system or an intermediate electron system between the itinerant electron system and the localized electron system has been difficult to be handled.

[0007] The band calculation method can be easily applied to an extremely small crystal system such as iron or nickel crystals and oxides thereof. However, even though the band calculation method is applied to a small crystal system, an enormous calculation quantity is required. Therefore, the following problem is left. That is, for example, it is substantially difficult that the band calculation method is applied to an actual huge crystal system such as a neodymium-based rare earth magnetic material (Nd₂Fe₁₄B crystal) from the viewpoint of calculation quantity.

[0008] On the other hand, in consideration of the circumstances, a method of easily predicting ferromagnetism by changing molecular orbitals is proposed (Japanese Patent Application Laid-Open No. H11-6825). However, magnetic characteristics detected in an experiment cannot be calculated using this method, and a subject to be calculated is limited to an organic high polymer compound. For this reason, the method poses a problem in a practical use.

SUMMARY OF THE INVENTION

[0009] It is an object of this invention to obtain a method of calculating a magnetic interaction in not only an itinerant electron system but also a localized electron system or an intermediate electron system across both the electron systems.

[0010] The magnetic interaction calculation method according to one aspect of this invention comprises calculating a magnetic interaction of molecules using a localized magnetic orbital η_(a) which satisfies a maximum overlap condition $\begin{matrix} {{\sum\limits_{\quad a}^{\quad}\quad \left| {\langle\left. \eta_{a} \middle| \omega_{a} \right.\rangle} \right|} = {{maximum}\quad,\quad {wherein}}} \\ {\eta_{a} = {\sum\limits_{i}^{\quad}\quad {T_{ai}\varphi_{i}}}} \end{matrix}$

[0011] and ω_(a) is a reference orbital given by $\omega_{a} = {\sum\limits_{i}^{\quad}\quad {c_{ai}^{- 1}\varphi_{i}}}$ where $\varphi_{i} = {\sum\limits_{a}\quad {c_{ai}\chi_{a}}}$

[0012] is a molecular orbital expressed as a linear combination of an atomic orbital χ_(a), c_(ai) is an expansion coefficient, c_(ai) ⁻¹ is an inverse matrix of the expansion coefficient c_(ai), and T_(ai) is the expansion coefficient of the a-th orbital η_(a) for the i-th molecular orbital φ_(i).

[0013] The magnetic interaction calculation method according to another aspect of this invention comprises calculating a magnetic interaction of crystalline compounds or high polymer compounds using a localized magnetic crystal orbital η_(a)(k) which satisfies a maximum overlap condition $\begin{matrix} {{\sum\limits_{a}\quad \left| {\langle\left. \eta_{a} \middle| \omega_{a} \right.\rangle} \right|} = {{maximum}{\quad,}\quad {wherein}}} \\ {{\eta_{a}(k)} = {\sum\limits_{i}\quad {{T_{ai}(k)}{\varphi_{i}(k)}}}} \end{matrix}$

[0014] and ω_(a)(k) is a reference orbital given by ${\omega_{a}(k)} = {\sum\limits_{i}\quad {{c_{ai}(k)}^{- 1}{\varphi_{i}(k)}}}$ where ${\varphi_{i}(k)} = {\sum\limits_{a}\quad {{c_{ai}(k)}{\sum\limits_{R}\quad {{\exp \left( {\quad {kR}} \right)}{\chi_{a}(R)}}}}}$

[0015] is a crystal orbital expressed as a linear combination of an atomic orbital χ_(a), c_(ai) (k) is an expansion coefficient, c_(ai) (k)⁻¹ is an inverse matrix of the expansion coefficient c_(ai) (k), T_(ai)(k) is the expansion coefficient of the a-th localized magnetic orbital η_(a)(k) for the i-th crystal orbital φ_(i)(k), k is a wave vector ranging from −π/c to π/c where c is primitive vectors, and R is a vector representing crystal periods.

[0016] The magnetic characteristic value calculation method accordingto still another aspect of this invention comprises calculating a magnetic characteristic value based on each of the magnetic interaction calculation methods.

[0017] The program according to still another aspect of this invention causes, when executed on a computer, the computer to execute each of the magnetic interaction calculation methods.

[0018] The program according to still another aspect of this invention causes, when executed on a computer, the computer to execute each of the magnetic characteristic value calculation methods. Each of the magnetic characteristic value calculation methods comprises calculating a magnetic characteristic value based on each of the magnetic interaction calculation methods.

[0019] These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a diagram which shows a unit cell structure in an Nd₂Fe₁₄B crystal used in calculation when the magnetic interaction calculation method of the present invention is evaluated,

[0021]FIG. 2 is a comparative graph which shows Co-dopant quantity dependence of a Curie temperature in Nd₂(Fe_(1−x)Co_(x))₁₄B,

[0022]FIG. 3 is a comparative graph which shows Co-dopant quantity dependence of a residual magnetic flux density in Nd₂(Fe_(1−x)Co_(x))₁₄B, and

[0023]FIG. 4 is a comparative graph which shows Dy-dopant quantity dependence of residual magnetic flux density in an (Nd_(1−x)Dy_(x))₂Fe₁₄B crystal.

DETAILED DESCRIPTION

[0024] Embodiments of the magnetic interaction calculation method or the magnetic characteristic value calculation method according to the present invention will be described below with reference to the accompanying drawings. It should be noted that the present invention is not limited to this embodiment.

[0025] Molecular orbitals φ_(i) can be expressed as a linear combination of atomic orbitals χ_(a) by the following equation: $\varphi_{i} = {\sum\limits_{a}\quad {c_{ai}\chi_{a}}}$

[0026] Where reference symbol c_(ai) denotes an expansion coefficient, and satisfies the following standardizing condition: $\begin{matrix} {{\sum\limits_{a}\quad {\sum\limits_{b}\quad {c_{ai}^{T}s_{ab}c_{bi}}}} = 1} & (1) \end{matrix}$

[0027] where S_(ab) is an overlap integral between χ_(a) and χ_(b).

[0028] Because the molecular orbitals φ_(i) are canonical orbitals to establish a standard orthogonal system (perfect system), localized magnetic orbitals η_(a) can be developed as expressed in the following equation (2) by using φ_(i) as a base: $\begin{matrix} {\eta_{a} = {\sum\limits_{i}\quad {T_{ai}\varphi_{i}}}} & (2) \end{matrix}$

[0029] It is assumed that, as a sum in the equation (2), φ_(i) is taken about occupied orbitals when the localized magnetic orbitals η_(a) are occupied orbitals (η_(a) ^(occ)), and φ_(i) is taken about unoccupied orbitals when the localized magnetic orbitals η_(a) is unoccupied orbitals η_(a) ^(vac).

[0030] A reference orbital ω_(a) which is localized on the atomic orbitals χ_(a) is defined by using an inverse matrix c_(ai) ⁻¹ of the expansion coefficient c_(ai) as follows: $\omega_{a} = {\sum\limits_{i}\quad {c_{ai}^{- 1}\varphi_{i}}}$

[0031] In this equation, the reference orbitals ω_(a) are an asymptotic solution. The inverse matrix c_(ai) ⁻¹ can be calculated from the equation (1).

[0032] The localized magnetic orbitals η_(a) can be obtained from the reference orbitals ω_(a) and the next maximum overlap condition: $\begin{matrix} {{\sum\limits_{a}\quad \left| {\langle\left. \eta_{a} \middle| \omega_{a} \right.\rangle} \right|} = {maximum}} & (3) \end{matrix}$

[0033] In addition, the equation (3) is resolved to a maximum problem of Ω(T) expressed by the following equation (4): $\begin{matrix} \begin{matrix} {{\Omega (T)} = {\sum\limits_{a}\quad \left| {\langle\left. \eta_{a} \middle| \omega_{a} \right.\rangle} \right|^{2}}} \\ {= {\sum\limits_{a}\quad \left| {\sum\limits_{i}\quad {T_{ai}*{\langle\left. \varphi_{i} \middle| \omega_{a} \right.\rangle}}} \right|^{2}}} \\ {= {\sum\limits_{a}\quad \left| {\sum\limits_{i}\quad {\sum\limits_{j}\quad {T_{ai}*c_{aj}^{- 1}{\langle\left. \varphi_{i} \middle| \varphi_{j} \right.\rangle}}}} \right|^{2}}} \\ {= {\sum\limits_{a}\quad \left| {\sum\limits_{i}\quad {T_{ai}*c_{ai}^{- 1}}} \right|^{2}}} \end{matrix} & (4) \end{matrix}$

[0034] As is apparent from the equation (4), the localized magnetic orbitals η_(a) can be expressed as a subspace of the inverse matrix c_(ai) ⁻¹. Therefore, when a subspace matrix of the inverse matrix c_(ai) ⁻¹ defined by the equation (4) is represented by d_(ia), according to the equation (2), the localized magnetic orbitals η_(a) can be expressed with respect to occupied orbitals η_(a) ^(occ) and unoccupied orbitals η_(a) ^(vac) given by the following equations: $\begin{matrix} {\eta_{a}^{occ} = {\left( {\sum\limits_{i}^{occ}\quad d_{ia}^{2}} \right)^{1/2}{\sum\limits_{i}^{occ}\quad {d_{ia}\varphi_{i}}}}} \\ {\eta_{a}^{vac} = {\left( {\sum\limits_{i}^{vac}\quad d_{ia}^{2}} \right)^{{- 1}/2}{\sum\limits_{i}^{vac}\quad {d_{ia}\varphi_{i}}}}} \end{matrix}$

[0035] The occupied orbitals η_(a) ^(occ) and the unoccupied orbitals η_(a) ^(vac) have one-to-one correspondence (isomorphism mapping) with respect to the reference orbitals ω_(a) localized in the atomic orbitals χ_(a).

[0036] An electron count L_(a) ^(occ) and a hole count L_(a) ^(vac) held by the occupied orbitals η_(a) ^(occ) and the unoccupied orbitals η_(a) ^(vac) are given by the following equations, respectively: $\begin{matrix} {L_{a}^{occ} = {\sum\limits_{i}^{occ}\quad d_{ia}^{2}}} \\ {L_{a}^{vac} = {\sum\limits_{i}^{vac}\quad d_{ia}^{2}}} \end{matrix}$

[0037] When the orbital energies of the occupied orbitals η_(a) ^(occ) and the unoccupied orbitals η_(a) ^(vac) are represented by λ_(a) ^(occ) and λ_(a) ^(vac), respectively, the orbital energies λ_(a) ^(occ) and λ_(a) ^(vac) are given by the following equations: $\begin{matrix} {\lambda_{a}^{occ} = {\left( {\sum\limits_{i}^{occ}\quad d_{ia}^{2}} \right)^{{- 1}/2}{\sum\limits_{i}^{occ}\quad {d_{ia}ɛ_{i}}}}} \\ {\lambda_{a}^{vac} = {\left( {\sum\limits_{i}^{vac}\quad d_{ia}^{2}} \right)^{{- 1}/2}{\sum\limits_{i}^{vac}\quad {d_{ia}ɛ_{i}}}}} \end{matrix}$

[0038] where å_(i) is an orbital energy of the molecular orbitals φ_(i).

[0039] According to the above equations, an ionization potential IP_(a) and an electron affinity EA_(a) in the localized magnetic orbitals can be expressed from the orbital energy λ_(a) ^(occ) and the orbital energy λ_(a) ^(vac) as follows: $\begin{matrix} {{IP}_{a} = {- \lambda_{a}^{occ}}} \\ {{EA}_{a} = {- \lambda_{a}^{vac}}} \end{matrix}$

[0040] In the above description, the molecular models are explained. However, since the present invention can be similarly extended to corresponding crystal orbitals in a crystalline compound or a high polymer compound, a description thereof will be omitted.

[0041] A magnetic interaction based on localized magnetic orbitals will be described below. An effective exchange interaction J_(ab) ^(eff) representing a magnetic interaction is approximated by the sum of a direct exchange interaction J_(ab) ^(EX) and a super exchange interaction J_(ab) ^(SE). J_(ab)^(eff) = J_(ab)^(EX) + J_(ab)^(SE)

[0042] The direct (potential) exchange interaction J_(ab) ^(EX) based on the localized magnetic orbitals is expressed by the following equation: J_(ab)^(EX) = 2K_(ab)L_(a)^(occ)L_(b)^(occ)

[0043] In this equation, an exchange integral K_(ab) is given by the following equation: $K_{ab} = {\int{\int{{\eta_{a}^{*}(1)}{\eta_{b}^{*}(2)}\frac{1}{r_{12}}{\eta_{a}(2)}{\eta_{b}(1)}{\tau_{1}}{\tau_{2}}}}}$

[0044] and L_(a) ^(occ) and L_(b) ^(occ) denote the numbers of electrons of the occupied orbitals η_(a) ^(occ) and η_(b) ^(occ) in the localized magnetic orbitals.

[0045] A super exchange interaction J_(ab) ^(SE) based on the localized magnetic orbitals is expressed by the following equation: J_(ab)^(SE) = K_(ab)^(SE)L_(a)^(occ)L_(b)^(vac)

[0046] In this equation, by using a transfer integral t_(ab) and an overlap integral s_(ab), a kinematic integral K_(ab) ^(SE) is given by the following equation: K_(ab)^(SE) = 2t_(ab)S_(ab)

[0047] According to the above description, a model which calculates a magnetic interaction can be achieved. Therefore, a magnetic characteristic value is then calculated by using the model, and is compared with an experimental value to be verified. Here, as the magnetic characteristic value, a Curie temperature or a residual magnetic flux density are evaluated.

[0048] In a first embodiment, in order to examine the reliability and versatility of the present invention, the physicality value of a neodymium-based rare earth magnet was evaluated. The neodymium-based rare earth magnet is an intermediate electron system having itinerant electrons and localized electrons. A unit cell in the crystal of the electron system consists of 68 atoms. This electron system is a huge system which cannot be easily calculated by a conventional band calculation method.

[0049]FIG. 1 is a diagram which shows a unit cell structure in an Nd₂Fe₁₄B crystal used in calculation. Here, the theoretical value of a Curie temperature obtained when Fe is displaced with Co is compared with a calculated value based on the present invention. The Curie temperature is a target to be evaluated because the Curie temperature is an important measure with which an effective exchange interaction energy obtained from localized magnetic crystal orbitals is compared with the experimental value. Since Fe can be completely replaced with Co, the calculation result and the experiment result can be effectively compared with each other by a replacement ratio (dopant ratio) of Co. The effect of the dopant of Co is considered by using imaginary crystal approximation which is a kind of mean field approximation.

[0050] More specifically, the Curie temperature in an Nd₂(Fe_(1−x)Co_(x))₁₄B crystal was evaluated. FIG. 2 is a comparative graph which shows Co-dopant quantity dependence of the Curie temperature in Nd₂(Fe_(1−x)Co_(x))₁₄B. The experiment result provided here is quoted from a reference “Y. Matsuura et al. Appl. Phys. Lett., 46, 308 (1985)”. As is apparent from FIG. 2, the Curie temperature with respect to the Co-dopant quantity exhibits the same trend in the experiment and the calculation.

[0051] In particular, in an area in which the Co-dopant quantity is 25% or less, the experimental value and the calculated value of the Curie temperature are almost equal to each other. Therefore, it was confirmed that the Curie temperature calculated from the present invention could be “quantitatively” compared with the experiment result. More specifically, it was confirmed that an effective exchange interaction energy based on the present invention was reliable.

[0052] Since it was understood by the present invention that the Curie temperature was quantitatively approximate to an actually measured value, simulation for Co-dopant dependence of a residual magnetic flux density was performed in a similar system. FIG. 3 is a comparative graph which shows Co-dopant dependence of a residual magnetic flux density in Nd₂(Fe_(1−x)Co_(x))₁₄B. The experiment result provided here was measured at 4.2 K, and is quoted from a reference “S. Hirosawa et al. J. Appl. Phys., 59,873 (1986)”. Only the experiment result related to Nd₂Fe₁₄B is described because the residual magnetic flux density of the Nd₂(Fe_(1−x)Co_(x))₁₄B crystal is not reported.

[0053] As shown in FIG. 3, the experimental value and the calculated value of a residual magnetic flux density in Nd₂Fe₁₄B were 18.50 kG and 16.60 kG, respectively. It was confirmed that these values were almost equal to each other. As shown in FIG. 3, the residual magnetic flux density decreases as the Co-dopant quantity increases. This downward trend is the same as that of a result related to a saturated magnetic flux density. Therefore, a calculated value (physicality value) taken along the experimental value can be extremely effectively obtained based on the method of calculating a magnetic interaction according to the present invention.

[0054] Although the calculation based on the present invention was performed by an engineering workstation, the computation time was 50 minutes. Since it was estimated that the band calculation method based on the conventional density functional theory required at least a computation time of about several weeks, it was confirmed that magnetic characteristics could be easily calculated by the calculation method according to the present invention.

[0055] In a second embodiment, as in the first embodiment, comparative evaluation of a residual magnetic flux density of a Dy₂Fe₁₄B crystal was performed. In addition, a calculation was also performed to an (Nd_(1−x)Dy_(x))₂Fe₁₄B crystal. FIG. 4 is a comparative graph which shows a Dy-dopant dependence of the residual magnetic flux density in the (Nd_(1−x)Dy_(x))₂Fe₁₄B crystal. The experiment result provided here is quoted from the reference “S. Hirosawa et al. J. Appl. Phys., 59,873 (1986)”.

[0056] As shown in FIG. 4, the experimental value and the calculated value of a residual magnetic flux density in Dy₂Fe₁₄B were 5.73 kG and 8.48 kG, respectively. When this calculated value is compared with the calculated value in Nd₂Fe₁₄B, it is understood that the calculated value can be quantitatively compared with the experimental value. The computation time required for this calculation was55 minutes. Since it was estimated that the band calculation method based on the conventional density functional theory required at least a computation time of about several weeks, it was confirmed that magnetic characteristics could be easily calculated by the calculation method according to the present invention.

[0057] The embodiments according to the present invention have been described above. The concrete example of configuration is not limited to the embodiments, and the design of the configuration may be changed without departing from the spirit and scope of the invention.

[0058] For example, a computer executable program which causes a computer to execute the magnetic interaction calculation method or the magnetic value calculation method, and this program is recorded on a computer readable recording medium. The program recorded on the recording medium is loaded on the computer to be executed, so that the respective functions may be realized.

[0059] As described above, according to the present invention, the magnetic interaction in not only the itinerant electron system but also the localized electron system or the intermediate electron system across both the electron systems can be advantageously calculated by using the localized magnetic orbitals or the localized magnetic crystal orbitals. Therefore, the magnetic characteristic value in the localized electron system or the intermediate electron system across the both electron systems can be advantageously calculated. Furthermore, the magnetic characteristic values of large molecules, a crystalline compound, and a high polymer compound can be easily calculated and predicted advantageously.

[0060] Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth. 

What is claimed is:
 1. A magnetic interaction calculation method comprising: calculating a magnetic interaction of molecules using a localized magnetic orbital η_(a) which satisfies a maximum overlap condition ${\sum\limits_{a}^{\quad}\quad \left| {\langle\left. \eta_{a} \middle| \omega_{a} \right.\rangle} \right|} = {{maximum},}$ wherein $\eta_{a} = {\sum\limits_{i}\quad {T_{ai}\varphi_{i}}}$

 and ωa is a reference orbital given by $\omega_{a} = {{\sum\limits_{i}{c_{ai}^{- 1}\varphi_{i}\quad {where}\quad \varphi_{i}}} = {c_{ai}\chi_{a}}}$

 is a molecular orbital expressed as a linear combination of an atomic orbital χ_(a), c_(ai) is an expansion coefficient, c_(ai) ⁻¹ is an inverse matrix of the expansion coefficient c_(ai), and T_(ai) is an expansion coefficient of the a-th orbital η_(a) for the i-th molecular orbital φ_(i).
 2. The method according to claim 1, wherein as the magnetic interaction, an orthogonal exchange interaction 2K_(ab)L_(a) ^(occ)L_(b) ^(occ) obtained by a potential exchange integral K_(ab) in localized magnetic orbitals η_(a) and η_(b) and electron counts L_(a) ^(occ) and L_(b) ^(occ) of occupied orbitals, is employed.
 3. The method according to claim 1, wherein as the magnetic interaction, a super exchange interaction K^(SE) _(ab)L_(a) ^(occ)L_(b) ^(vac) obtained by a kinematic exchange integral K^(SE) _(ab) in localized magnetic orbitals η_(a) and η_(b), an electron count L_(a) ^(occ) of occupied orbitals, and a hole count L_(b) ^(vac) of unoccupied orbitals, is employed.
 4. A magnetic interaction calculation method comprising: calculating a magnetic interaction of crystalline compounds or high polymer compounds using a localized magnetic crystal orbital η_(a)(k) which satisfies a maximum overlap condition $\begin{matrix} {\sum\limits_{a}{{{{\langle{\eta_{a}{\omega_{a}\rangle}}} = {maximum}},}}} \\ {{{wherein}{\quad \quad}{\eta_{a}(k)}} = {\sum\limits_{i}{{T_{a\quad i}(k)}{\varphi_{i}(k)}}}} \end{matrix}$

 and ω_(a)(k) is a reference orbital given by $\begin{matrix} {{\omega_{a}(k)} = {\sum\limits_{i}{{c_{ai}(k)}^{- 1}{\varphi_{i}(k)}\quad {where}}}} \\ {{\varphi_{i}(k)} = {\sum\limits_{a}{{c_{ai}(k)}{\sum\limits_{R}{{\exp \left( {\quad k\quad R} \right)}{\chi_{a}(R)}}}}}} \end{matrix}$

 is a crystal orbital expressed as a linear combination of an atomic orbital χ_(a), c_(ai) (k) is an expansion coefficient, c_(ai) (k)⁻¹ is an inverse matrix of the expansion coefficient c_(ai) (k), T_(ai)(k) is an expansion coefficient of the a-th localized magnetic orbital η_(a)(k) for the i-th crystal orbital φ_(i)(k), k is a wave vector ranging from −π/c to π/c where c is primitive vectors, and R is a vector representing crystal periods.
 5. The method according to claim 4, wherein as the magnetic interaction, an orthogonal exchange interaction 2K_(ab)L_(a) ^(occ)L_(b) ^(occ) obtained by a potential exchange integral K_(ab) in localized magnetic crystal orbitals η_(a)(k) and η_(b)(k) and electron counts L_(a) ^(occ) and L_(b) ^(occ) of occupied orbitals, is employed.
 6. The method according to claim 4, wherein as the magnetic interaction, a super exchange interaction K^(SE) _(ab)L_(a) ^(occ)L_(b) ^(vac) obtained by a kinematic exchange integral K^(SE) _(ab) in localized magnetic crystal orbitals η_(a)(k) and η_(b)(k), an electron count L_(a) ^(occ) of occupied orbitals, and a hole count L_(b) ^(vac) of unoccupied orbitals, is employed. 